Add RST syntax check

[felixfontein]

Addressing #49 (comment)

[briantist]

Alright looks like I got it; I think warning is the right level of things to fail on, because if we let warnings through, it would fail for the strictest settings in the rest of the build.

And the original thing about the links not being referenced was info level so I think this works out perfectly?
Will let you confirm when you've got time.

[felixfontein]

Feel free to merge :)

[briantist]

Thanks for your continued work on this @felixfontein !

[felixfontein]

Thanks for polishing and merging this!